Did you install the alignment rings between the intake manifold runners and the cylinder head? If they were not installed, it's very possible the gasket slipped during installation and you've got a major vacuum leak. Try putting a stream of gas from a Propane torch (not lit) around the intake-to-head area and see if the engine smooths out or increases speed. Some folks use starting fluid or carburetor cleaner for this check, but Propane or Acetyline is a lot less messy.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!